The present invention relates to ceramic cutting tool materials and particularly to such cutting tool materials in which monocrystalline whiskers (hair crystals) together with small (significantly less than 1 .mu.m) particles are uniformly distributed in a ceramic matrix which leads to an increased strength and toughness without negatively influencing the wear resistance of the material.
Ceramic cutting tool materials have been available for several decades. However, until recently they have not had any significant commercial importance for use in chipforming machining. The main reason for the limited growth of ceramic cutting tools has been sudden and unexpected tool failures because of their inherent inadequate strength and toughness.
In recent years, the properties of ceramic cutting tool materials have been improved in many respects and their use in cutting of cast iron and heat-resistant alloys (e.g., nickel-base alloys) has relatively increased. The proportion of ceramic cutting inserts is still very small, however, where steel is the dominating work piece material being machined because steel makes large simultaneous demands upon strength, toughness and wear resistance which have not been fulfilled by currently known ceramic cutting tool materials.
Aluminum oxide-based cutting tool materials are very sensitive to thermal crack formation because aluminum oxide in itself has a relatively poor thermal conductivity. This leads to very short tool lives in machining steel, particularly under conditions with short operating times and varying cutting depth.
To a certain extent, the thermal properties have been improved by additions of titanium carbide and/or titanium nitride which enhance the thermal conductivity of the tool material. The addition of titanium carbide/nitride also increases the hardness of the material. In comparison with pure aluminum oxide materials, an increased tool life is therefore obtained in the cutting of harder work piece materials and in operations demanding thermal shock resistance. However, this kind of material has too poor a toughness behavior for a more general use in the cutting of steel.
A later development relates to alloying of uniformly dispersed fine-grained zirconium oxide particles in a matrix of aluminum oxide. A transformation of the `metastable` zirconium oxide particles during use increases both strength and toughness and thus leads to a more predictable tool life.
The thermal properties of said type of materials are, however, only slightly better than those of pure aluminum oxide materials. Therefore, initiation and growth of thermally induced cracks is still a great problem in practical cutting operations generating high curing edge temperatures such as cutting of steel.
It has recently been shown (T. N. Tiegs and P. F. Belcher, J. Am. Ceram. Soc. 90(5) C-109-C-11, 1987) that alloying of SiC-whiskers, with monocrystalline hair crystals, in a matrix of aluminum oxide leads to a greatly improved fracture toughness and strength. Ceramic cutting tool materials based upon said concept have shown very good performance in the cutting of heat-resistant materials in particular but in the cutting of steel they have shown surprisingly short tool lives because of preferential attack of the SiC-crystals. This leads to a weakening of the surface zone with accompanying high wear and risks of crack initiation.
U.S. Pat. No. 4,867,761 discloses oxide-based ceramic cutting tool materials strengthened by whiskers of carbides, nitrides and borides of Ti and Zr or solid solutions thereof having a low solubility in steel resulting in a cutting tool material with an improved and more predictable toughness as well as improved strength and resistance to thermal shocks without deterioration of the wear resistance to any appreciable degree particularly when machining steel. This has not been possible with earlier known material compositions.
In U.S. Pat. No. 5,141,901 further improvements have been achieved using whiskers of nitrides, carbides and/or borides of Ta. These whiskers have a much lower thermal expansion coefficient than the alumina matrix material which leads to further improvements of toughness and thermal shock resistance. The mechanisms are not known but depend probably on a favorable situation with respect to internal stresses in the composite material.
It has recently been found (M. Kanamaru, T. Tatsuno and T. Kasuka, "Hot Pressed Al.sub.2 O.sub.3 /SiC Whisker/TiC Nano-Composites", Journal of The Ceramic Society of Japan, 100(4), 408-412, 1992) that further improvements of the properties are possible in the whisker-reinforced cutting tool materials, especially the strength, if small additions of nanosize particles are added to the whisker reinforced materials.
It is well-known that particulate additions can be used to improve the properties of a brittle ceramic material, for example, U.S. Pat. No. 4,320,203 which refers to additions of TiN and Ti(C,N).
Depending on the nature of the additions the operating toughening mechanisms can be crack deflection, microcracking, transformation toughening or crack bridging. It is characteristic for these particulate additions that the size of the particles are of the same order of magnitude as the matrix material e.g., in the order of 1-5 .mu.m and that they are located in the grain boundaries of the matrix material. Only in the case of ZrO.sub.2 -additions is a smaller grain size than the matrix needed in order to suppress a transformation during fabrication of the material. However, the zirconia particles are still predominantly located in the grain boundaries of the alumina matrix material.
It is also characteristic for the particulate reinforced materials that property improvements are achieved up to rather high particulate contents, normally up to 15-30 percent by volume.